Study Of Terahertz Wakefield Radiation Based On Accelerators

During the last decades,terahertz radiation has played a more and more important role in frontier technology fields such as life science,material characterization,infor-mation transmission,and security detection,and its development has also promoted the growth of terahertz radiation sources.As an one of THz radiation sources,the tera-hertz wakefield which excited by the relativistic electron beam from the accelerators in dielectric loaded waveguide(DLW)has attracted more and more researchers due to DLW’s advantages of high breakdown gradient,small volum and low cost.This the-sis explores and investigates some areas in high-power terahertz wakefield radiation sources based on accelerators,include principle of wakefield excitation,structure op-timization,diffraction and propagation of terahertz wakefield,development of related numerical calculation methods,and compact terahertz wakefield radiation experiments,etc.In order to satisfy the demand for high power radiation sources in some terahertz technologies,two new DLW structures are proposed in this paper,and the properties of the terahertz wakefield excited inside them are analyzed with help of theoretical and simulation methods.The first structure is a radially separated dielectric loaded waveg-uide,in which the peak power of the wakefield is increased by adding the distance be-tween the dielectric layer and the metal layer.According to the results,the peak power of the excited wakefield in the new structure is increased by more than one order of that in the normal DLW.The metal layer of another structure is tilted with a certain nega-tive angle,the gradient of the wakefield excited by relativistic electron bunch will be substantially increased,which we call negatively tapered dielectric loaded waveguide.Compared with the normal DLW,the gradient of the wakefield excited in this structure is increased by more than 4 times.For the problem of large diffraction loss of the wakefield at the export of the DLW,we propose to use the special relationship between the new dielectric layer and the elec-tron velocity in the double-layer DLW to expand the export aperture without changing the mode frequency of the wakefield and intrinsically reduce the divergence angle of the field from the structure.The results show that this scheme can reduce the diffraction divergence angle of the wakefield by one-third and the diffraction loss from more than 80%to less than 10%.In addition,thesis proposes to feed the terahertz wakefield into the optical resonant cavity,so that the individual mode in the wakefield can be picked out and amplified to improve the monochromaticity of the field.To solve the problem of low efficiency of PIC simulation in above scheme,we propose a semi-analytic fast PIC numerical method,which can significantly reduce the simulation time assumed by PIC software to calculate the beam-field coupling process,and it contains a one-dimensional program to calculate the high-energy electron beam-field interaction and a two-dimensional program for non-relativistic electron beam-field interaction.A graphical interactive interface has been developed to further reduce the difficulty of using this numerical method.And based on the Stratton-Chu diffraction equations,we propose a simple method to numerically calculate the propagation of the terahertz wakefield,which can reduce the difficulty in field propogation between three-dimensional asymmetric optical elements.At last,this thesis introduces the design and preparation of a desktop high power terahertz wakefield radiation source experiment based on a high harmonic type terahertz wakefield free electron laser scheme.The design and testing of the thermionic cathode electron gun,solenoid and vacuum chamber have been finished and the preparation of optical elements have also been completed.The experiment device is compact and can excite high-power wakefield radiation in three modes within the 0.1 THz～0.3 THz range.